In astronomy, a spicule is a short-lived (about 5 minutes), narrow jet of gas, less than 10,000 kilometers long and 500 kilometers in diameter, in the upper chromosphere of the Sun. Visible in the light of H-alpha, spicules tend to cluster at the edges of supergranulation cells and are usually seen as a mass of tiny brighter spike-like features at the limb or as tiny darker spikes coming out of network elements. Macrospicules are about 10 times larger and are best seen in the He II ultraviolet line at 304 Å.

In zoology, a spicule is a crystalline or mineral deposit found in sponges, sea cucumbers, or urochordates. Spicules are structural components in many sponges, and may serve a protective function in other organisms.